Soil carbon cycling responses to elevated CO2 and nitrogen addition

Abstract

Soils are the largest terrestrial pool of carbon (C). Understanding the mechanisms responsible for C loss from soils, and how they might respond to global change, is therefore necessary for predicting future C-climate feedbacks. However, it remains uncertain how soil C processes will respond to an increasingly changing global environment, marked by rising carbon dioxide (CO2) emissions and nitrogen (N) deposition from human activities. While decades of experiments have provided insight into how inputs of organic matter to soils might respond to elevated CO2 and N deposition, there is still considerable uncertainty around how global change will affect losses of C from soils. Therefore, this dissertation assesses the effects of elevated CO2 and N addition on three distinct C cycling processes that collectively contribute to the net release of C from soils: 1) soil aggregation, a key control of soil organic matter (SOM) accessibility to decomposers, 2) microbial decomposition of SOM in bulk soil, and 3) priming of SOM decomposition by roots and their associated microbial communities. To do this, we developed a suite of field- and lab-based experiments that built on the frameworks of two long-term global change experiments at the Cedar Creek Ecosystem Science Reserve in central Minnesota. Overall, we found that global change could result in distinct responses in specific soil C cycling that could counteract each other. Specifically, elevated CO2 increased aggregation, hence reducing accessibility of SOM to decomposers, likely due to both bacterial and fungal activity. C loss from microbial respiration in the absence of roots consistently did not respond to long-term N addition, across different ecosystem types, which contradicts current thinking that N addition may inhibit microbial decomposition and lead to greater accumulation of C in soils. Finally, soil C loss from microbial decomposition in the rhizosphere increased, on average by 34-39%, with elevated CO2, likely due to increased C inputs to rhizosphere soils, whereas it decreased by 29%, on average, with N addition. Collectively, these findings highlight the importance of assessing multiple processes at play in soil C cycling, as individual mechanisms might not reveal the actual response in total soil C loss.